There are various experiments that you cane do with candles for the since fair about the burning time. For example you can compare the burning time of same sized but one white and one colored candle. You can also compare the burning time of scented candles with normal unscented candles. You can compare the burning time of refrigerated candle with the one kept at room temperature. Another thing that you can do is measure the height of the flame and then determine at what time does it burn the brightest. You can also try to measure how long a candle burns when you cut off the air by covering it with a glass or jar.